Open-type magnet device for MRI

ABSTRACT

An open-type magnet device for MRI includes a pair of upper and lower magnet assemblies, and shims are provided on opposite sides of their cooling containers and/or in holes passing through central portions of the cooling containers so as to facilitate providing a highly homogeneous static magnetic field.

TECHNICAL FIELD

The present invention relates to an open-type magnet device to be usedfor MRI (magnetic resonance imaging) and in particular, to an open-typemagnet device which can appropriately be used to obtain a highly-uniformstatic magnetic field.

BACKGROUND ART

In magnet device of horizontal magnetic field type or in a magnet deviceof vertical magnetic field type using a pole piece, a magnetic shim isused for improving uniformity of a static magnetic field generated bythe magnet device.

However, in the magnet device of the vertical magnetic field type notusing any pole piece, the uniformity of the static magnetic fieldgenerated by the magnet device has been improved by an adjusting coilprovided in the magnet device.

DISCLOSURE OF INVENTION

It is therefore an object of the present invention to provide anopen-type magnet device using a magnetic shim enabling to obtain afurther highly-uniform magnetic fields and facilitating adjusting workfor that.

Another object of the present invention is to provide an oper-typemagnet device in which a space required for arranging a magnetic shim isreduced. As a result, it is possible to increase a space for a subjectperson or reduce the distance between upper and lower superconductivemagnets arranged to oppose to each other, thereby improving theefficiency of superconductive magnets.

Still another object of the present invention is to provide an open-typemagnet device in which the magnetic shim is arranged in a regionassuring its magnetization linearlity, thereby facilitatingoptimalization of the magnetic shim position and enabling an effectivemagnetic field uniformity adjustment work.

The present invention uses an open-type superconductive magnet equippedwith a magnetic shim (shims) which serves as means for adjustingmagnetic field uniformity and is arranged in a pair of magneticassemblies and more specifically, on an opposing surface side of acooling vessel, i.e., a uniform static magnetic field space side.

As is described in JP-A-9-153408 and JP-A-9-190913, in a magnetassembly, a static magnetic field generating coil is composed of a maincoil having a largest diameter arm an adjusting oil having a smallerdiameter. These coils are arranged coaxially around a Z axis. In thisconfiguration, magnetization of the magnetic shim in a region where themagnetic shim is arranged, i.e., magnetic vector direction is mostlyvertical, i.e., Z direction in the vicinity of the cooling vessel centerline (Z axis) and become vertical to the Z axis as the distance from theZ axis becomes greater. This means that in the vicinity of the Z axis,the magnetic shim is almost uniformly magnetized not depending on theposition of the magnetic shim. That is, by arranging the magnetic shimin a region of a small radius from the center of the cooling vessel, itis possible to expect a constant magnetization. Accordingly, whencalculating the position of the magnetic shim, there is no need ofconsidering magnetization of the respective magnetic shims, therebysimplifying shimming work.

A region where the radius direction component of the shim magnetizationis increased chances depending on a specific magnet assembly. However,when roughly observed, in a region of 0 to ⅔ of the main coil radiusfrom the center, magnetization components are mostly Z-directioncomponents.

However, even in a region having a greater radius, by actual measuringand computer simulation, it is possible to check the relationshipbetween the position of the magnetic shim and its magnetization and todetermine the position of the magnetic shim according to the calculateddata. This is because the magnetic shim has a large magnetization changein the radius direction but a small magnetization change in acircumferential direction of a circle around the Z axis. That is,superconductive coils serving as the magnetic field generating source ismade from a plurality of coaxial shapes and basically generates amagnetic field symmetric with respect to the axis. On the other hand,the magnetic shield provided at the coil outer circumference has anon-symmetric shape with respect to the Z axis because of the presenceof a yoke and the like. This non-symmetric shape of the magnetic shielddoes not affect so much to the position of the magnetic shim introducedby the present invention. It has been confirmed that the magnetic shimis magnetized with identical components in any position in thecircumferential direction. Accordingly, even when using an outercircumference portion having a large radius direction component, it isoften sufficient to consider shim magnetization change amount dependingon the position in the radius direction. Furthermore, when a highuniformity of a magnetic field is required, it is sufficient to fetch ashim magnetization change in the circumferential direction by datainterpolation according to the uniformity degree.

In the open-type superconductive magnet disclosed in the afcrementionedJP-A-9-153408 and JP-A-9-190913, it has been confirmed that by arrangingthe magnetic shim on the opposing surface of the cooling vessel, it ispossible to adjust the magnetic field uniformity. Moreover, a positionwhere the magnetic shim is actually arranged may be other than theuniform static magnetic field space side of the cooling vessel, such asthe uniform static magnetic field space side of the gradient magneticfield coil. In case the gradient magnetic field coil is magneticallyshielded, the shim may be arranged between the main gradient magneticfield coil and the gradient magnetic field shield coil or/and at theuniform magnetic field space side of the main gradient magnetic fieldcoil.

According to the present invention, it is possible to obtain anopen-type magnet device in which magnetic field adjusting means isprovided at the opposing surfaces of cooling vessels sandwiching anuniform static magnetic field space region constituting an upper and alower magnet assemblies, which enables to further improve uniformity ofthe static magnetic field formed therebetween.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows an external perspective view of an open-typemagnet device according to the present invention.

FIG. 2 is longitudinal cross sectional view of the open-type magnetdevice of FIG. 1.

FIG. 3 is a perspective view showing arrangement of magnetic shimsaccording to an embodiment of the present invention.

FIG. 4 is a longitudinal cross sectional view of FIG. 3.

FIG. 5 is a perspective view showing arrangement of magnetic shimsaccording to another embodiment of the present invention.

FIG. 6 is a longitudinal cross sectional view of FIG. 5.

FIG. 7 is a perspective view showing arrangement of magnetic shimsaccording to still another embodiment of the present invention.

FIG. 8 is a perspective view showing arrangement of magnetic shimsaccording to yet another embodiment of the present invention.

FIG. 9 is a perspective view showing arrangement of magnetic shimsaccording to yet still another embodiment of the present invention.

FIG. 10 is a perspective view showing arrangement of magnetic shimsaccording to yet another embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Description will now be directed to an embodiment of the presentinvention wish reference to FIG. 1, FIG. 2, FIG. 3, and FIG. 4.

As shown in FIG. 1 and FIG. 2, an open-type magnet device 10 has a pairof an upper magnet assembly 12 and a lower magnet assembly 14 whichoppose to each other. Each of the upper and lower magnet assemblies 12and 14 includes: a main superconductive coil 18 for generating a uniformstatic magnetic field space 16 therebetween; an adjustingsuperconductive coil 20 for adjusting uniformity of a static magneticfield generated by the main superconductive coil 18; a coolant vessel 22containing a coolant for cooling the main superconductive coil 18 andthe adjusting superconductive coil 20 to a temperature of the superconductivity or below and containing the superconductive coils; a vacuumvessel 24 made from a non-magnetic material such as aluminum andstainless steel and entirely covering the coolant vessel 22 so as toprevent heat convection; the coolant vessel 22 and the vacuum vessel 24constituting a cooling vessel; and a ferromagnetic plate 26 covering theoutside of the cooling vessel so as suppress leak magnetic flux from thesuperconductive coils. The superconductive coils 18 and 20 are arrangedcoaxially around a Z-axis as a center. The coolant vessel 22 and thevacuum vessel 24 of each of the upper and lower magnet assemblies 12 and14 are connected to each other by a connection tube 28. The upper andlower ferromagnetic plates 26 are supported by a yoke 30 andmagnetically connected. A reference symbol 41 denotes a main gradientmagnetic coil and 42 denotes a shield gradient magnetic field coil.

Furthermore, as shown in FIG. 3 and FIG. 4, one or more than onemagnetic shim 51 is arranged in the opposing planes of the vacuum vessel24 serving as a cooling vessel of the upper and lower magnet assemblies12 and 14. FIG. 3 shows the lower magnet assemblies 14.

A specific shape of the magnetic shims 51 arranged in this region, forexample, may be a cube of 10 mm×10 mm×10 mm or a ring shape centered atthe Z-axis.

Moreover, the magnetic shims may be made from, for example, known softmagnetic material such as iron, silicon steel, permalloy, and the like.

FIG. 5 and FIG. 6 show another embodiment of the present invention. Inthis embodiment, the vacuum vessel 24 serving as a cooling vessel ismade in a doughnut shape having a through hole 34 at the center portionthereof, ard the magnetic shim 51 is arranged in an opposing surface 32or/and a region 35 of a through hole. That is, in addition to, theregion 32 shown in FIG. 4, the magnetic shim is arranged in a centerhole portion 35, i.e., on a wall of this portion and a support member.

This center hole portion 35 is in the vicinity of the center axis of thesuperconductive coil and accordingly, in general, the magnetic fielddistribution becomes almost uniform. Therefore, not depending on theposition of the magnetic shim 51, shim magnetization is constant. Thisfacilitates optimal arrangement performed upon shimming. Thus, a rangeof shim positions for adjusting the magnetic field uniformity isincreased by using the magnetic shim in the enter hole 34 in addition tothe magnetic shims arranged on the opposing surface of the coolingvessel. This simplifies adjustment of the magnetic field uniformity.

Moreover, since the magnetic shim can be located at a positiondifference in distance in the Z direction from the uniform magneticfield, a wider selection range can be obtained for selecting a ratiobetween various magnetic field components, if expressed in sphericalharmonics function generated by the magnetic shimming, such ascoefficients of each of terms of first degree, second degree, thirddegree . . . . This facilitates approach of coefficients of the firstdegree and above to zero. This enables to cope with various changes ofthe ratio of the magnetic components to be controlled.

Moreover, for a component adjustable by magnetic shims arranged on theopposing surface of the cooling vessel, use of the shim in the centerhole can reduce the shim amount on the opposing surface. Thus can reducethe space of the region on the opposing surface where shims arearranged, which in turn can increase a uniform magnetic space into whicha examined sample is inserted. Alternatively, this can reduce theopposing distance between the superconductive magnets and accordingly,can improve the magnetic field generation efficiency of the magnet,which in turn can reduce the entire size of the magnet device.

It should be noted that JP-A-7-250819 also discloses a configuration inwhich a magnetic shim is arranged along an inner surface of a centerhole provided in a cooling vessel. However, this center hole isconfigured so as to receive a subject person and accordingly, the shimcan be arranged only on a surface portion along the inner circumference.Accordingly, the ability of the uniformity adjustment is also limited.On the other hand, the present invention assumes a magnet in which asubject person is not positioned in the center hole and the magneticshim arrangement is not limited to a particular position of the centerhole, thereby enabling to obtain uniformity adjustment of a wider range.

FIG. 7 shows still another embodiment of the present invention.

In the embodiment of FIG. 5 and FIG. 6, the shims on the opposingsurface are not located at the center portion so as not to disturbmounting of the center hole shim.

In contrast to this, in the embodiment of FIG. 7, in order toeffectively use the center portion of the opposing surface, a magneticshim region 36 is also provided in this center portion. The magneticshim 51 in this region is attached to a non-magnetic mounting jig andarranged at least detachably. For attaching/removing the center shim,what is necessary is only to remove the center portion of the opposingsurface. Therefore, working ability is not deteriorated.

FIG. 8 shows yet another embodiment of the present invention.

In this embodiment, the shim region of the opposing surface is limitedto a constant radius r from the center. In the region at a smallerradius direction distance from the center, main component ofmagnetization in the magnetic shim arrangement regions 32, 35, 36 is Zcomponent. Accordingly, magnetization of magnetic members becomesuniform and the shims can be arranged optimally. Moreover, work resultscoincide calculated values and it is possible to reduce the number ofshimming processes.

The radius limit can be selected to obtain a range in whichmagnetization has a main component in Z direction. This range is optimalwhen 0 to ⅔ of a diameter of the main superconductive coil. In general,the dimension from the inner wall of the outer circumference of thecoolant vessel 22 to the outermost circumference of the vacuum vessel 24is 60 mm, to 150 mm. On the other hand, when considering an MRI systemfor an entire body of a person, the main superconductive coil preferablyhas radius of 700 to 1000 mm. On the other hard, in order to minimizethe outer dimension of the magnet, the outermost circumference of themain superconductive coil is arranged in the vicinity of the inner wallof the outer circumference of the coolant vessel 22. Moreover, thevacuum vessel 24 is also made as small as possible in a range where theheat intrusion into the coolant vessel 22 is not greater than apredetermined value.

Accordingly, the radius limit is about ⅔ of the cooling vessel, i.e.,the outer diameter of the vacuum vessel 24.

FIG. 9 shows yet another embodiment of the present invention.

FIG. 9 shows an example where, in addition to the regions 32, 35, and 36of FIG. 8, the magnetic shims 51 are also arranged in a region 37 of theuniform static magnetic field space side of the main gradient magneticfield coil 41 and a region between the main gradient magnetic field coil41 and the main shield gradient magnetic field coil 42.

The magnetic shims need not be arranged in all of the regions 32, 35,36, 37 and 38 but can be arranged at least one of the regions, so as toadjust uniformity of the magnetic field.

FIG. 10 shows still another embodiment of the present invention in whichthe gradient magnetic field coil is not shielded. In this case, theshield gradient magnetic coil 42 is not used and only the main gradientmagnetic field coil 41 exists. In this case, in addition to the regions32, 35, and 36, the region 37 of the uniform static magnetic field spaceside of the main gradient magnetic field coil 41 becomes a candidate asa region where the magnetic shim is arranged. In this case also, themagnetic shim may be provided at one or more than one of the regions 32,35, 36, and 37 for adjusting uniformity of the magnetic field.

While FIG. 9 and FIG. 10 show examples in combination with FIG. 8, theycan also be used in combination of embodiments of FIG. 4, FIG. 6, andFIG. 7. In these cases also, the magnetic shim may be provided at one ormore than one of the regions 32, 35, 36, 37 and 38 for adjustinguniformity of the magnetic field.

In each of the embodiments of FIG. 4, FIG. 6, FIG. 7, and FIG. 8 also,the magnetic shim is arranged in one or more than one regions 32, 35,and 36 so as to adjust uniformity of the magnetic field.

In FIG. 4, FIG. 6, and FIG. 7 to FIG. 10, explanation has been given onarrangement of the magnetic shims in the magnet assembly 14. Identicalshim arrangement regions are also provided in the magnet assembly 12 andthe magnetic shims are arranged in these regions for adjustinguniformity of the magnetic field.

The present invention can also be applied to an MRI apparatus using aresistive magnet. In this case, the MRI apparatus has configuration asfollows.

An open-type magnet device comprising:

-   a pair of upper and lower magnetic field generating coils arranged    to oppose each other so as to generate a uniform static magnetic    field in a region which covers a test portion of a subject person;-   a container for containing each of the magnetic field generating    coils; and    -   at least one magnetic shim arranged at the uniform static        magnetic field space region side of the respective containers        and serving to further adjust magnetic field uniformity of the        uniform static magnetic field space region.

(2) The open-type magnet device for MRI according to (1), wherein thecontainers are doughnut-shaped having a through hole in the centerportion and the magnetic shim is arranged at a predetermined position inthe through hole.

(3) The open-type magnet device for MRI according to (1), wherein agradient magnetic field coil is arranged at the uniform static magneticfield space region side of each of the containers and the magnetic shimis arranged at the uniform static magnetic field space region side ofthe gradient magnetic field coil.

The present invention is not limited to the aforementioned embodimentsbut may be modified in various way within the scope of claims.

INDUSTRIAL APPLICABILITY

The present invention can be applied not only an MRI apparatus using asuperconducting magnet device or static magnetic field generating coilbut also to an MRI apparatus using a permanent magnet device.

1. An open-type magnet device for MRI, comprising: a pair of an upperand a lower magnet assemblies which are arranged to oppose to each otherso as to generate a uniform static magnetic field in a space regionwhich covers a test portion of a subject, and each of which includes: amain superconductive coil for generating a uniform static magnetic fieldtherebetween and an adjusting superconductive coil for adjustingmagnetic field uniformity of the uniform static magnetic field; acooling vessel containing the main superconductive coil and theadjusting superconductive coil for maintaining a superconductive state;and magnetic field adjusting means arranged at the uniform staticmagnetic field space side of the cooling vessel, so as to further adjustthe magnetic field uniformity of the uniform static magnetic field spaceregion; wherein the cooling vessel is a doughnut-shaped cooling vesselhaving a through hole in a center portion thereof and the magnetic fieldadjusting means is arranged at a predetermined position of the throughhole; and the magnetic field adjusting means is also arranged to theside of the uniform static magnetic field space and outside the throughhole.
 2. An open-type magnet device for MRI, comprising: a pair of anupper and a lower magnet assemblies which are arranged to oppose to eachother so as to generate a uniform static magnetic field in a spaceregion which covers a test portion of a subject, and each of whichincludes: a main superconductive coil for generating a uniform staticmagnetic field therebetween and an adjusting superconductive coil foradjusting magnetic field uniformity of the uniform static magneticfield; a cooling vessel containing the main superconductive coil and theadjusting superconductive coil for maintaining a superconductive state;and magnetic field adjusting means arranged at the uniform staticmagnetic field space side of the cooling vessel, so as to further adjustthe magnetic field uniformity of the uniform static magnetic field spaceregion; wherein the cooling vessel is a cylinder-shaped or adoughnut-shaped cooling vessel having a through hole in a center portionthereof and the magnetic field adjusting means is arranged in a regionrange of 2R/3 wherein R represents a radius of the opposing surface ofthe cooling vessel.
 3. An open-type magnet device for MRI, comprising: apair of an upper and a lower magnet assemblies which are arranged tooppose to each other so as to generate a uniform static magnetic fieldin a space region which covers a test portion of a subject and each ofwhich includes: a main superconductive coil for generating a uniformstatic magnetic field therebetween and an adjusting superconductive coilfor adjusting magnetic field uniformity of the uniform static magneticfield; a doughnut-shaped cooling vessel having a through hole in acenter portion thereof and containing the main superconductive coil andadjusting superconductive coil for maintaining a superconductive state;and magnetic field adjusting means arranged at a predetermined positionin the through hole of the cooling vessel, so as to further adjustmagnetic field uniformity of the uniform static magnetic field spaceregion; wherein the magnetic field adjusting means is also arranged tothe side of the uniform static magnetic field space and outside thethrough hole.
 4. The open-type magnet device for MRI as claimed in claim3, comprising at least one additional magnetic field adjusting meansarranged between the uniform static magnetic field space and the coolingvessel.
 5. The open-type magnet device for MRI as claimed in claim 3,wherein at least one of the magnetic field adjusting means is alsoarranged along a cooling vessel side-wall which faces a centrallongitudinal axis of the through hole of the cooling vessel.
 6. Theopen-type magnet device for MRI as claimed in claim 3, comprisingadditional ones of the magnetic field adjusting means provided ondiffering layers arranged between the uniform static magnetic fieldspace and the cooling vessel.
 7. The open-type magnet device for MRI asclaimed in claim 3, wherein at least one of the magnetic field means isdetachably attached to the inside or outside of the through hole.